A fiber optic security system monitors an area which the system installed, and discerns whether any intrusion or attempt to intrude is or not. It is installed on such facilities as an armed force, airports, power plants, detention houses etc. to protect the facilities from possible intruders. It can reduce the number of guards and has the advantage of providing a substitutive means of communication using the optical fibers installed.
There are some methods of using optical fibers as a sensor; method for measuring dynamic variation of optical signals by using the optical speckle, method for measuring static variation of the strength of optical signals due to optical losses, and method for measuring physical fluctuation of optical fibers by using the back scattering of optical signals.
A security system using the optical speckle to detect an intruder discerns whether there is any intrusion by cutting off some part of optical signals at an end of the optical fibers and measuring the strength of the optical signals passed. Because there occur multipath interferences caused by changes of environment in an optic fiber during the transfer of the optical signals through the optical fiber, wherein the optical signals are transferred in the form of continuous waves.
And there is a method of using the back scattering of optical signals generated at the optical fibers during the transfer of optical signals through the optical fibers. This method discerns whether there is any intrusion by detecting the strength of returning optical signals which are back-scattered at transferring media transferring in the form of pulses. This method is able to detect even the location where the physical fluctuation occurs at the optical fiber.
FIG. 7 and FIG. 8 are illustrative diagrams of fiber optic security systems disclosed as US and EP patents; FIG. 7 is disclosed in U.S. Pat. No. 5,592,149 of Uri Alizi and FIG. 8 is disclosed in european patent No. 0,049,979 of Pilkington P. E. Ltd.
As shown in the figures, the security systems each comprises a support(120) set up at the place where need to be secured in a predetermined shape, a net of optical fibers(130) fixed to said support(120), a transmitter controller(100) transmitting predetermined optical signals to said net of optical fibers(130), and a receiver controller(110) discerning whether intruded or not by analyzing the optical signals received from said net of optical fibers(130).
Said net of optical fibers(130) is in the form of interlooped optical wire structure formed by simply crossing and twisting a strand of the optical fiber.
The security system having the above constitution detects a signal of external intrusion while said transmitter controller(100) transmits optical signals to said net of optical fibers(130) and said receiver controller receives the optical signals passed through said net of optical fibers(130).
Namely the security system, when someone cuts or stretch said net of optical fibers(130), detects change of optical signals.
The security system having said net of optical fiber is not cost-effective to construct and convenient to install because said net of optical fiber should be weaved at places installed one by one to meets the size of the area to be protected. Furthermore, the security system has the problem of high expense of maintenance because said net of optical fiber should be replaced entirely when it is cut off.
In addition, the security system has the problem of malfunction caused by the fact that it can not discriminate between the change of the optical signals on account of attempt to intrude and the change of the optical signals on account of natural phenomena such as rain, wind, or vibration taken place in neighborhood.
The present invention is conceived to solve the problems of the conventional fiber optic security system. Accordingly, it is an object of the present invention to provide a fiber optic security system being simple and easy to install regardless of the size of the area to be protected, and convenient to maintain and a control method thereof.
Another object of the present invention is to provide a fiber optic security system having transparent fibers to be seen outer scenery through when installed, having a fine appearance, reducing false alarms caused by external environmental change such as rain, vibration or the wind, and improving reliability of the system by means of preventing hackings and a control method thereof.